A radio communication system compliant with LTE (Long Term Evolution) in 3GPP (Third Generation Partnership Project) is known. This radio communication system is configured by arranging a plurality of base stations so that each of the base stations communicates with a communication terminal (a mobile station) located within a communication area (referred to as a cell hereinafter) allocated to the base station.
The radio communication system uses the same communication band in each of a plurality of cells. Therefore, a difference between the level of a signal transmitted and received by a communication terminal (referred to as an edge terminal hereinafter) located on the border between cells to and from the base station of an own cell (a cell to which the edge terminal belongs) and the level of a signal (i.e., an interference signal) transmitted and received in an adjacent cell (a cell adjacent to the own cell) is small. Thus, there is a fear that the quality of a communication path (the communication path quality) between the edge terminal and the base station of the own cell becomes extremely low (deteriorates).
In order to address such a problem, there is a known technique called FFR (Fractional Frequency Reuse) aiming at suppression of signal interference between cells. The FFR is a technique of limiting allocation of a radio resource (a communication band and transmission power) in the adjacent cell in order to secure the quality of a communication path between the edge terminal and the base station of the own cell.
Herein, the outline of an operation of a radio communication system in which the FFR is applied to a downlink (a communication link for transmitting data from a base station to a communication terminal) will be described. In this example, as shown in FIG. 1, the radio communication system is equipped with three base stations 1 to 3 and nine communication terminals 11 to 13, 21 to 23, and 31 to 33.
To each of the base stations 1 to 3, one cell is allocated. To be specific, a cell C1 is allocated to the base station 1, a cell C2 is allocated to the base station 2, and a cell C3 is allocated to the base station 3. Each of the base stations may be configured so that a plurality of cells can be allocated thereto.
Further, the communication terminals 11 to 13 belong to the cell C1 (i.e., a communication link for performing communication with the base station 1 is established). The communication terminals 21 to 23 belong to the cell C2. The communication terminals 31 to 33 belong to the cell C3. Herein, a case that the communication terminals 12, 13, 21, 23, 31 and 32 are edge terminals and the other communication terminals 11, 22 and 33 are center terminals will be assumed.
For the respective cells, the radio communication system sets priority bands that vary with the cell. In this example, as shown in FIG. 2, the radio communication system divides a communication band (a system band) F0 available in the radio communication system into three partial bands F1 to F3, sets the partial band F1 as a priority band of the cell C1, sets the partial band F2 as a priority band of the cell C2 and sets the partial band F3 as a priority band of the cell C3.
Next, each of the communication terminals notifies communication path quality information representing the communication path quality to the base station. Based on the notified communication path quality information, the base station determines whether the communication terminal having notified the communication path quality information is a terminal (referred to as the edge terminal hereinafter) on which an influence of signal interference from the adjacent cell is comparatively large or a terminal (referred to as the center terminal hereinafter) on which an influence of signal interference from the adjacent cell is comparatively small.
After that, the base station allocates a communication band to be used for performing communication with the edge terminal from the set priority band. Therefore, describing with the base station 1 as an example, as shown in FIG. 3, an edge terminal allocatable band FE that is a communication band allocatable as a communication band to be used for radio communication between the base station 1 and the edge terminal is set to the priority band F1 of the cell C1.
Moreover, as shown in FIG. 4, the base station uses preset reference transmission power P0 as transmission power to be used for performing communication with the edge terminal. For example, the reference transmission power P0 is an average value over the whole system band F0 of the maximum values of power that the base station can simultaneously output for transmission of radio signals.
Further, the base station allocates a communication band to be used for performing communication with the center terminal from the whole communication band available in the cell (i.e., the system band). Therefore, describing with the base station 1 as an example, as shown in FIG. 3, a center terminal allocatable band FC that is a communication band allocatable as a communication band to be used for performing radio communication between the base station 1 and the center terminal is set to the system band F0.
Furthermore, as shown in FIG. 4, the base station uses limitation transmission power P1 that is smaller than the reference transmission power P0 by a preset transmission power difference ΔP, as the transmission power to be used for performing communication with the center terminal.
According to this, since the interference of radio signals transmitted and received within another cell in radio signals using the priority band is suppressed, it is possible to improve the communication path quality between the edge terminal and the base station (Non-Patent Document 1).
The base station determines a communication band to be allocated to each of the communication terminals based on the allocated transmission power. In the LTE, a unit of allocation of a communication band is called a resource block (RB). The base station determines an actually allocated RB (an allocation RB) from the allocatable communication band for each of the communication terminals.
Further, the base station determines a modulation and coding scheme (MCS) representing a combination of a modulation scheme and a code rate, based on the number of the determined allocation RBs and channel quality information (CQI) reported from the communication terminals.
CQI is information obtained by quantizing the communication path quality of a channel such as a data channel, and is defined by a table in the LTE specification (Non-Patent Document 2). In the CQI table, a relation between a communication band and a modulation scheme, a code rate and spectrum efficiency for achieving a target error rate is specified. Index (information for identifying data within the table) in this table is set in the ascending order from data of low spectrum efficiency.
Because a combination of a modulation scheme and a code rate is previously set in the CQI table, it is possible to previously calculate a communication path quality (SINR: a signal to noise interference ratio) necessary for achieving the target error rate. The communication terminal measures the communication path quality and reports, to the base station, the Index of CQI with the highest spectrum efficiency in a range that the target error rate can be achieved.
Further, the base station stores a table in which the number of the allocation RBs and a data size (TBS: a transport block size) that can achieve the target error rate is related. Like CQI, this table is defined by the LTE specification (Non-Patent Document 2). Moreover, when the TBS is determined, the modulation scheme is also determined from another table (Non-Patent Document 2). Therefore, it is possible to previously calculate a required communication path quality for each TBS.
In the case of transmitting data to the communication terminals by using a plurality of RBs, the base station uses the same modulation scheme for all of the RBs. Therefore, the base station calculates an average communication path quality from the Index of the CQI reported from the communication terminals, and determines the TBS based on the calculated communication path quality.
To be specific, the base station selects the largest TBS from TBSs that can achieve a required error rate based on the calculated communication path quality. The base station selects a TBS smaller than a TBS necessary for transmitting unsent data. Moreover, the Index in the TBS table is notified to the communication terminal as an MCS Index.    [Non-Patent Document 1] Bin Fan, et al., “A Dynamic Resource Allocation Scheme Based on Soft Frequency Reuse for OFDMA Systems,” IEEE 2007 International Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, IEEE, August 2007, pp. 121-125    [Non-Patent Document 2] 3GPP TS 36.213 V8.8.0, September 2009, pp. 25-33, 47-48
As transmission power used for transmitting a radio signal to a certain communication terminal decreases, spectrum efficiency (each-terminal spectrum efficiency), which is the amount of information that can be transmitted to the communication terminal per unit time and per unit communication band, also decreases.
Therefore, the spectrum efficiency of the whole base station (all-terminal spectrum efficiency) becomes lower when the limitation transmission power P1 is allocated as the transmission power for a center terminal than when the reference transmission power P0 is allocated as the transmission power for a center terminal. Herein, the all-terminal spectrum efficiency is an average value of the each-terminal spectrum efficiency over all of the communication terminals belonging to the own cell.
For example, the following case will be assumed: in the example shown in FIGS. 1 to 4, the base station 1 allocates the priority band F1 as a communication band and allocates the reference transmission power P0 as transmission power to an edge terminal and allocates the partial bands F2 and F3 and allocates the limitation transmission power P1 as transmission power to a center terminal.
In this case, as shown in FIG. 5, the transmission power allocated by the base station 1 is the reference transmission power P0 in the partial band F1, and is the limitation transmission power P1 in the partial bands F2 and F3. Therefore, in the partial bands F2 and F3, surplus power PA corresponding to the transmission power difference ΔP occurs. That is to say, in this example, there is a problem that the all-terminal spectrum efficiency wastefully lowers for the surplus power in the base station 1.
Accordingly, an object of the present application is to provide a base station capable of solving the aforementioned problems.